Active Matrix Organic Light Emitting Diode (AMOLED) displays have been widely used due to their wide viewing angle, good color contrast effect, high response speed, low cost and other advantages. However, a drift in threshold voltage may result from the problem of non-uniformity of a Thin Film Transistor (TFT) array substrate in a process flow.
As illustrated in FIG. 1, a traditional 2T1C pixel circuit includes a switch transistor T1, a drive transistor T2, a storage capacitor C1 and an Organic Light Emitting Diode (OLED). A scan signal, denoted Scan, is received at the gate of the switch transistor T1, and the scan signal Scan includes a signal on a gate line connected with the pixel circuit. An image data signal, denoted Data, is received at the source (or the drain) of the switch transistor T1, the drain (or the source) of the switch transistor T1 is connected with a first end of the storage capacitor C1, a first drive signal VDD is received at a second end of the storage capacitor C1, the first drive signal VDD is received at the source of the drive transistor T2, the gate of the drive transistor T2 is connected with the first end of the storage capacitor C1, the drain of the drive transistor T2 is connected with a first end of the OLED, and a second drive signal VSS is received at a second end of the drive transistor T2. When a start signal in the scan signal Scan is received at the gate of the switch transistor T1, the switch transistor T1 is turned on, and the image data signal Data received at the source (or the drain) of the switch transistor T1 is transferred to the drain (or the source) of the switch transistor T1 and stored in the storage capacitor C1, wherein the drive transistor T2 is controlled by the image data signal Data together with the first drive signal VDD to operate so that the OLED is driven by the current at the drain of the drive transistor T2 to emit light. In such a 2T1C pixel circuit, the current at the drain to drive the OLED to emit light is dependent upon the threshold voltage of the drive transistor T2, and with a long operating time a drift in threshold voltage of the drive transistor T2 may result from the characteristic of the transistor per se, thereby varying a current flowing through the organic light emitting diodes in an array of pixel circuits, and imposing a direct influence upon the brightness of the light emitting diodes, which will be more apparent in a high-power light emitting diode display element.
In summary, a drift in threshold voltage of a drive transistor in a pixel element over its operating period of time may have the same OLED driven by varying current to emit light upon reception of the same image data signal during different periods of time so that the brightness of the OLED will vary over its operating period of time.